This invention relates to fuel injection control for preventing combustion noise in a transient running state of a diesel engine.
Tokkai Hei 6-346763 published by the Japanese Patent Office in 1994 and Japanese Patent No. 2864896 issued by the Japanese Patent Office in 1998 disclose low-temperature premixing combustion performed by a diesel engine in order to reduce noise and smoke produced by the operation thereof.
This low-temperature premixing combustion is applied specifically to a diesel engine for a vehicle that has a low compression ratio ranging from 14 to 18 and is provided with an exhaust gas recirculation system, a turbocharger and a swirl control valve.
In a diesel engine designed to perform the low-temperature premixing combustion, fuel ignition timing is largely retarded to lengthen the ignition delay period of injected fuel so that the fuel injection is terminated and the injected fuel is fully mixed with air within this ignition delay period. Further, large amounts of exhaust gas recirculation (EGR) is conducted so that the resultant fuel mixture is burnt gradually. On the other hand, an excess air factor of the fuel mixture is maintained by turbocharging and the premixing is enhanced by a swirl control valve.
The low-temperature premixing combustion as described above is effective in suppressing the generation of noise and smoke in a steady running state of the engine, but results in the following problems in a transient running state such as when the engine is accelerating.
The operation mechanism of the engine during acceleration may be described as follows.
Firstly the fuel injection amount is increased and the exhaust pressure and exhaust temperature are increased as a result of combustion of increased fuel. The energy of the exhaust gas drives the exhaust gas turbine of the turbocharger and the compressor of the turbocharger which is driven by the exhaust gas turbine increases the fresh air intake amount into the engine.
Due to a turbo-lag of the turbocharger, a differential pressure upstream and downstream of the EGR valve is temporarily increased by the decrease in the intake pressure during the initial period of acceleration. Thus the exhaust gas recirculation amount is also temporarily increased. With the increase in the exhaust gas recirculation amount, the excess air factor of the gaseous fuel mixture in the engine is reduced and the ignition delay period of injected fuel may undergo an abnormal increase.
When the ignition delay period undergoes an abnormal increase, since the fuel is rapidly combusted after ignition, it is not possible to realize low-temperature premixing combustion. As a result, a large combustion noise is generated and exhaust gas composition is adversely affected.
It is therefore an object of this invention to prevent increases in noise and to prevent adverse effects on exhaust gas composition in the transient running state of the diesel engine.
In order to achieve the above object, this invention provides a fuel injection control device for such a diesel engine that comprises a piston reciprocating through a compression top dead center and a fuel injection mechanism of which a fuel injection amount and a fuel injection timing is controllable. The fuel injection control device comprises a sensor which detects a parameter expressing either of oxygen concentration and oxygen amount in gas aspirated into the diesel engine, and a programmable controller. The controller is programmed to calculate a difference between a parameter value set for a steady running state of the diesel engine and the parameter detected by the sensor. The controller is further programmed to control the fuel injection mechanism, when the difference is greater than a predetermined value, to cause the fuel injection mechanism to perform a main fuel injection at a first timing which completes a combustion of an injected fuel before the compression top dead center and a pilot fuel injection at a second timing earlier than the first timing. The controller is further programmed to control the fuel injection mechanism, when the difference is smaller than the predetermined value, to cause the fuel injection mechanism to perform a pilot fuel injection at a third timing with an injection amount which is larger than an injection amount of the pilot injection performed when the difference is greater than the predetermined value, and a main injection at a fourth timing, an interval between the third timing and the fourth timing being larger than a period required for combusting fuel injected by the pilot injection at the third timing.
This invention also provides a fuel injection control method for such a diesel engine that comprises a piston reciprocating through a compression top dead center and a fuel injection mechanism of which a fuel injection amount and a fuel injection timing is controllable. The fuel injection control method comprises detecting a parameter expressing either of oxygen concentration and oxygen amount in gas aspirated into the diesel engine, and calculating a difference between a parameter value set for a steady running state of the diesel engine and the parameter detected by the sensor. The method further comprises controlling the fuel injection mechanism, when the difference is greater than a predetermined value, to cause the fuel injection mechanism to perform a main fuel injection at a first timing which completes a combustion of an injected fuel before the compression top dead center and a pilot fuel injection at a second timing earlier than the first timing. The method further comprises controlling the fuel injection mechanism, when the difference is smaller than the predetermined value, to cause the fuel injection mechanism to perform a pilot fuel injection at a third timing with an injection amount which is larger than an injection amount of the pilot injection injection at a fourth timing, an interval between the third timing and the fourth timing being larger than a period required for combusting fuel injected by the pilot injection at the third timing.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.